The present invention relates to a method and apparatus of forming patterns of a thick film circuit or the like for use in forming of elements such as conductors and resistors on a thick film circuit board, and more particularly to a method and apparatus of forming patterns in thick film circuits or the like by a writing method which is capable of forming patterns of resistors and other elements at high speed and high precision.
Generally, a thick film circuit is one which has resistors, capacitors, conductors and other components formed on a relatively thick insulated board by depositing a relatively thin material layer on the insulated board. That is, as a first step, a conductor paste, which is a mixture of powder of silver-palladium compound or the like and glass frit dispersed in an organic solvent, is spread on an alumina ceramic board in a desired shape by a screen printing method. By heating at a relatively low temperature, the organic solvent in this conductor paste is dried and evaporated, and then the conductor paste is fired at a high temperature to form a conductor to be used as a wire. In a next step, a resistor paste of ruthenium oxide (RuO.sub.2) compound is applied on the board by a screen printing method in such a manner that it is partly overlapped with the conductor formed thereon and a resistor is formed by drying and firing in a method similar to forming the conductor.
Thus, the conventional forming method of thick film circuits required screens because the conductors and resistors were formed by use of a screen printing method, and the screens were replaced whenever a model was changed or a circuit pattern was modified. In contrast, a method of extruding a thick film paste from a nozzle, applying it on an alumina ceramic board and forming a desired conductor pattern or resistor pattern was disclosed at the 1978 International Microelectronics Symposium of ISHM, Minneapolis, Minn., Sept. 17, 1978, and was published in the journal, "Electronic Packaging and Production", January 1981, pages 98 to 109. This method does not require a screen because of the use of the pattern writing method by numerical control, and is advantageous in that the model change or circuit pattern modification may be effected instantly.
In this writing method, however, when forming patterns at a high speed, it has sometimes occurred that patterns could not be formed at the desired positions. That is, due to a response delay in an electric control system, response delay in an electromagnetic air valve, delay in pressure supply of air and other delays, one end of a resistor was not overlapped on a conductor, or two resistors contacted with each other. Accordingly, it was necessary to smooth the ends by slowing down the nozzle moving speed or scanning and writing while slightly deviating the position of the nozzle whose opening is small. Therefore, the writing method was useful for trial production of thick film circuits, but was not suited for mass production.